Field of the Invention
This invention relates to color cathode ray picture tubes, and is addressed specifically to an improved apparatus and method for use in the manufacture of tension mask rubes. The invention is useful in the manufacture of tension mask tubes of various types, including those used in home entertainment television receivers, and in medium-resolution and high-resolution tubes used in color monitors.
The tension mask is a part of the cathode ray tube front assembly, and is located in close adjacency to the faceplate. The front assembly comprises a faceplate including a screen having deposits of light-emitting phosphors, a shadow mask, and discrete shadow mask support means which extend from the faceplate. The support means, which may be composed of ceramic, metal, or a combination of both, are commonly termed "rails." The shadow mask is an apertured metallic foil which may, by way of example, be about 0.001 inch thick, or less. The mask must be supported in high tension a predetermined distance from the inner surface of the cathode ray tube faceplate, a dimension known as the "Q-distance."
The rails may comprise either four discrete sections or a single unitary structure. The design of the rails may conform to one of the several configurations described and claimed in U.S. Pat. No. 4,891,546, of common ownership herewith. The body of the rails shown is composed of a ceramic which is secured to the faceplate by devitrifying solder glass. A metal cap cemented to the ceramic provides a surface to which a flat tension mask can be welded. The cap also has a surface for deflecting a high-energy beam used to weld the mask to the cap.
If a cathode ray tube is to operate effectively, all gasses within envelope must be evacuated by heating the envelope to a high temperature for a predetermined period. At this stage of the production process, the tube is in a near-final stage of completion, that is, the screen has been deposited on the inner surface of the faceplate, the electron gun has been installed, and the faceplate assembly has been sealed to the funnel with devitrified solder glass.
In the evacuation process, the tube is exposed to a high temperature in a bake-out oven. The escape of gasses from the envelope as a result of the heating is assisted by a high-vacuum pump attached to a hollow glass tubulation that extends from the gun-end of the tube. When as near-perfect a vacuum as possible has been attained, the tubulation is sealed off, and the tube is allowed to cool. The final step in the evacuation process is to flash a "getter" within the envelope, which increases and maintains the vacuum level.
A certain percentage of the tubes will implode during the bake-out process. Such implosions are costly in terms of materials and labor, and every effort is made to hold the frequency of implosions to the smallest possible number.
A frequent cause of cracks leading to implosion was found to originate in the stresses in the faceplate induced at the ends of the mask-support rails that extend from the faceplate, and which are secured to the faceplate by devitrified solder glass. An analysis of the stresses induced in the faceplate by the rails is presented in application Ser. No. 458,129 filed Dec. 28, 1989, of common ownership herewith. The heating of the tube envelope during the bake-out cycle aggravates an otherwise latent stress condition, leading to the possible cracking of faceplates and consequent tube implosions.